Computers which support the modern civilization are operated by an electric current which is a flow of electrons. Devices which are applied to information record and erasure by control of this electric current are configured by semiconductors. The electrons flowing through the semiconductors are scattered by impurities and Coulomb's force to generate Joule heat.
For this reason, a cooling fan is required for the computer. Moreover, some of the input energy cannot be used for the information record and erasure because of the Joule heat, and an energy loss is caused. That is, there is no doubt that suppression of the scattering of electrons is a main technical development issue aiming at power saving of the electronic device.
Conventionally, one of solutions is a method of operating the electronic device at an extremely low temperature for suppressing the scattering of the electrons. For example, use of a superconductor corresponds to this method. Since the electron scattering becomes zero in the superconductor, neither electric resistance nor Joule heat is generated. Therefore, the electron scattering does not occur.
However, when this method is used, it is required to cool the electronic device down to several degrees of Kelvin, and therefore, the energy to be consumed for this cooling should be noted. Moreover, it is difficult to commonalize and practically use such an electronic device as using the extremely-low-temperature state. For this reason, there is a situation without acceptable means that can suppress the electron scattering at room temperature.
However, the situation has been changed since about 2007. This is because a theoretical model of a topological insulator has been proposed as a theory of physics. The topological insulator is an insulator holding a specific electron state occurring on a material surface or an interface thereof, and is explained based on a relativistic effect caused by motions of inner-shell electrons of an element having a speed close to the speed of light. The effect becomes larger with atomic number.
That is, by this feature of electrons (spin-orbit interaction), a term of the spin-orbit interaction cannot be neglected, and added to the Hamiltonian of the band structure formed by the electrons, so that the band structure and the energy eigen value are changed. At this time, in a specific material, the valence band top and conduction band bottom are coupled with each other, generating a conduction band at the surface with vacuum (or the interface with a normal insulator). On the other hand, inside the material, a band gap is formed in some cases.
As a result, an unusual physical property which has not been conventionally known is expected, the unusual physical property causing a conductor on the surface or the interface of the material while causing an insulator inside the material because of the presence of the band gap. A material having such a property is referred to as “topological insulator” (see Non-Patent Document 1).
The specific electron band structures of the topological insulator have such curious characteristics that electrons existing on the surface or interface of the material are degenerated with two different electron spin currents preserving the spin states by time-reversal symmetry. In other words, this means that the electron band structures have such a specific property as not causing the electron scattering by the impurities or others. Moreover, for example, if there is no such external magnetic field as disturbing the time-reversal symmetry, this property is preserved very tightly. Note that the naming of the topological insulator is derived from the fact that this property of the electron band structures has a nature similar to that of the topology polyhedral theory of mathematics (see Non-Patent Document 1).
Since the theoretical prediction of the presence of the topological insulator, the research for a material having this curious property has actually been started. As a result, a bismuth-tellurium alloy, an antimony-tellurium alloy and others having high crystallinity have been verified by experiments using a photoelectron spectroscopy. However, the single crystals of these materials used in the experiments are produced by a very slowly cooling from a molten alloy or others, and therefore, cannot be directly applied to the electronic device (see Non-Patent Document 2) in production.
Meanwhile, without any relation to the above-described topological insulator, in order to reduce power consumption of a phase-change solid-state memory, the present inventors have proposed a superlattice-type phase-change solid-state memory whose memory operation is achieved by forming a superlattice-type phase-change film obtained by laminating a crystal alloy layer made of germanium-tellurium and a crystal alloy layer made of antimony-tellurium so that the (111) crystal orientation plane axes and the c-axes of the respective crystal alloy layers are matched with each other, and by switching an arrangement structure of germanium atoms toward a crystal growth axis direction (see Patent Documents 1 and 2 and Non-Patent Document 1).
The present inventors have found out that this superlattice-type phase-change solid-state memory can be an ideal topological insulator. This is because, as shown in Non-Patent Document 1, while a crystal alloy layer (Sb2Te3 crystal alloy layer) having an antimony-tellurium atomic ratio of 2:3 is used for the topological insulator, a structure of arrangement of a plurality of the crystal alloy layers is just used for a recording layer of the superlattice-type phase-change solid-state memory in the proposal of the present inventors, the plurality of the crystal alloy layers being separated from each other by a crystal alloy layer (GeTe crystal alloy layer) having a germanium-tellurium atomic ratio of 1:1 with a band gap. However, it only should be verified whether or not the crystal alloy layer made of germanium and tellurium has the same function as that of the vacuum band. And, by a first principle calculation using quantum mechanics, it has been verified that this crystal alloy layer plays the same role as that of the vacuum band through simulations (see Patent Document 3).
According to the verification, at a certain point (gamma point) within the reciprocal lattice space, the conduction band bottom and the valence band top cross each other at one point so as to be made in contact with each other in the vicinity of Fermi band level with a Dirac cone. This phenomenon is a unique property of the topological insulator, and this gamma point just becomes a central symmetric point of the GeTe/Sb2Te3 superlattice. That is, it has been verified that this GeTe layers become non-scattering layers of the electrons where the electrons can be freely two-dimensionally moved in the superlattice (see Patent Document 3).
The present inventors have executed the above-described first principle calculations while changing the number of blocks of the GeTe crystal alloy layer (one block number is about 1 nm) and the number of blocks of the Sb2Te3 crystal alloy layer arranged above and below the GeTe crystal alloy layer, and then, have succeeded in actually manufacturing an artificial superlattice structure based on the calculation results using a sputtering apparatus (see Non-Patent Document 3).
Moreover, it has been verified that, when an external magnetic field is applied to a memory device having this superlattice structure, a very large magnetoresistance effect is caused at room temperature (see Non-Patent Document 4). This unique phenomenon is caused based on a Rashba effect of the superlattice structure by breaking the spatial inversion symmetry when electric field is applied for memory switching, and this Rashba effect is surprisingly larger than that of any magnetic material that has been conventionally known, and an energy difference in spin bands caused by the superlattice structure reaches as much as 200 eV. The magnetic resistance effect is as large as being capable of observing the difference in spin characteristics at room temperature (see Non-Patent Literature 4).
Furthermore, various types of the superlattice structures each having a different thickness of the Sb2Te3 crystal alloy layer are formed on silicon wafers, and a change in the electron spin density which is induced by applying an external magnetic field to these superlattice structures in a direction perpendicular a surface is measured as a change in reflectance by allowing circular polarized light to be incident thereto, and, as a result, it is verified that the Rashba effect is remarkably enhanced in the case of the Sb2Te3 crystal alloy layer having a thickness thinner than 2 nm, and that the difference in the reflectance caused by the spin splitting becomes smaller in the case of the thickness thicker than this value. In other words, it is concluded that the superlattice-type phase-change film having a thickness larger than this value has a small Rashba effect, and becomes the topological insulator (see Patent Document 3).
Incidentally, in order to effectively generate such electric and magnetic characteristics of the superlattice structure, it is required to grow and orient the crystal alloy layer made of Sb2Te3 or others and the crystal alloy layer made of GeTe or others while maintaining a common crystal axis.
As a method for obtaining the above-described oriented growth, a method of arranging an orientation control layer made of Sb2Te3 as a base of the superlattice structure has been proposed (see, for example, Patent Document 4 and Non-Patent Document 5). Moreover, a desirable temperature condition for forming the superlattice structure having the orientation control layer as the base has been proposed (see Non-Patent Document 5).